Chloral (CICCH3O) reacts with water to form the sedative and hypnotic agent chloral hydrate (CICCH(OH)2). Draw Lewis structures for each of the reactants and products in this reaction and calculate ΔH for the reaction in the gas phase given the bond energies below. Bond C-C C-H C-Cl O-H C-O C=O Bond Energy (kJ/mol) 347 414 339 467 358 745
The Correct Answer and Explanation is:
Correct Answer:
- Draw Lewis Structures:
- Chloral (Cl₃C–CHO):
- Cl₃C group: A carbon (central) single-bonded to three Cl atoms and one C (of CHO group)
- CHO group: The second carbon double-bonded to one oxygen and single-bonded to one hydrogen
- Chloral Hydrate (Cl₃C–CH(OH)₂):
- Cl₃C group remains the same
- Central carbon is bonded to one hydrogen and two OH groups
- Bond Counting:
Chloral Reactant:
- 1 C–C bond
- 1 C=O bond
- 1 C–H bond
- 3 C–Cl bonds
Water Reactant (H₂O):
- 2 O–H bonds
Total Bonds in Reactants:
- C–C: 1 × 347 = 347 kJ
- C–H: 1 × 414 = 414 kJ
- C–Cl: 3 × 339 = 1017 kJ
- C=O: 1 × 745 = 745 kJ
- O–H: 2 × 467 = 934 kJ
Total: 347 + 414 + 1017 + 745 + 934 = 3457 kJ
Chloral Hydrate Product:
- 1 C–C bond
- 1 C–H bond
- 3 C–Cl bonds
- 2 C–O bonds
- 2 O–H bonds
Total Bonds in Products:
- C–C: 1 × 347 = 347 kJ
- C–H: 1 × 414 = 414 kJ
- C–Cl: 3 × 339 = 1017 kJ
- C–O: 2 × 358 = 716 kJ
- O–H: 2 × 467 = 934 kJ
Total: 347 + 414 + 1017 + 716 + 934 = 3428 kJ
ΔH = Bonds broken – Bonds formed = 3457 – 3428 = +29 kJ
Explanation
In this problem, we are examining a chemical reaction between chloral (Cl₃C–CHO) and water (H₂O) that forms chloral hydrate (Cl₃C–CH(OH)₂). To determine the enthalpy change (ΔH) of the reaction in the gas phase, we use average bond energies. The enthalpy change is calculated using the formula:
ΔH = Total bond energy of bonds broken – Total bond energy of bonds formed
First, we identify and count the types of bonds in the reactants. In chloral, we have one carbon-carbon bond, one carbon-hydrogen bond, three carbon-chlorine bonds, and one carbon-oxygen double bond. Water adds two O–H bonds to the reactants.
Next, we examine chloral hydrate. Its structure has the same Cl₃C backbone, one carbon-hydrogen bond, two carbon-oxygen single bonds (replacing the original C=O), and two O–H bonds, now part of hydroxyl groups.
We then multiply each bond type by its given bond energy and sum these to get the total energy for the bonds in both the reactants and products. Subtracting the total bond energy of the products from that of the reactants gives us the net enthalpy change.
Our result, ΔH = +29 kJ, indicates that the reaction is slightly endothermic in the gas phase, meaning it absorbs a small amount of energy. This suggests that although the products are stable, the formation of chloral hydrate from chloral and water requires energy input when not in solution. This analysis is crucial for understanding reaction energetics and is especially relevant for pharmaceutical compound synthesis and application.
